The invention relates to a steam generator or boiler for use in a nuclear power station.
As is more particularly illustrated by FR-A-2 477 265, a steam generator equipping a nuclear power station conventionally comprises a vertically axed envelope, whose internal space is subdivided into two parts, by a horizontal plate, known as a tube sheet. The ends of the tubes of a bundle or nest of inverted U-tubes are fixed to the tube sheet and issue below the latter, respectively in an admission collector and a discharge collector for the water circulating in the primary circuit of the reactor, known as primary water. The water circulating in the secondary circuit of the reactor, known as secondary or feed water, is injected into the part of the steam generator located above the tube sheet. This feed water vaporizes on contact with the tubes as a result of the heat carried by the primary water. The resulting steam is extracted from the steam generator after successively traversing cyclone separators placed in substantially vertical pipes and then dryers located in the upper part of the envelope above the pipes containing the cyclone separators.
During installation or maintenance work on such a steam generator, it sometimes occurs that objects such as filler rods, screws, bolts, etc. are inadvertently introduced into the secondary circuit. It can also arise that objects-such as screws become detached during operation. When the feed water flows in the secondary circuit of the steam generator, all these objects constitute migrating bodies which may become jammed between the tubes of the bundle if they reach this part of the steam generator. This disturbs the flow of feed water and damages the tubes and can even lead to fracture thereof.
Part of the migrating bodies circulating in the secondary circuit of a steam generator comes from the feed water system and enters the steam generator by the feed water intake tube. Applicant's French Patent Application No. 91 14900 proposes a device making it possible to trap the migrating bodies introduced in this way into the steam generator.
Migrating bodies can also reach the nest of boiler tubes by pipes of the cyclone separators collecting the part where the tube nest is housed to the dryers. These migrating bodies coming from the area of the dryers are either objects such as screws which become detached from a dryer during operation, or objects introduced into the space between the upper plate of the cyclone separators and the dryers during an installation or maintenance intervention in the boiler, and inadvertently left behind in the latter. At present there is no device which is able to trap such migrating bodies, so that there is a risk of their falling to the bundle of tubes on passing through the substantially vertical pipes in which the cyclone separators are located.